Justification:
The range of the Eastern Pondmussel is restricted to eastern North America where it extends from the lower Great Lakes east through New York to New Hampshire and south, in coastal rivers, to South Carolina.

This species was previously regarded as Near Threatened (IUCN 1996), based on decline levels which were observed and considered to be close to meeting the Vulnerable level (20%, version 2.3).

Since 1996 the populations have continued to be impacted by the expansion of the Zebra Mussel throughout the range, such that it is now considered Federally Endangered in Canada, and listed as Endangered in two states and one province, Threatened in two states and special concern in two states throughout the range of L. nasuta.

In Canada over 90% of historical range for the species is now infested with Zebra Mussels, which has lead to a dramatic decline in the species range, where it now remains in two small, widely separated populations, one in the delta area of Lake St.Clair and one in a tributary of the upper St. Lawrence River (COSEWIC 2007).

The larger part of the range in USA has also seen large declines in population abundance of between 25-40%, between 1989 and 2011 in many rivers as well as extirpation from watersheds. The main cause is expansion of the Zebra Mussel throughout the river systems although other factors such as pollution, damming and dredging river channels has also impacted subpopulations. As such it is considered to have met the 30% decline threshold for Vulnerable, and may meet the Endangered threshold if declines continue.

The range of the Eastern Pondmussel is restricted to eastern North America where it extends from the lower Great Lakes east through New York to New Hampshire and south, in coastal rivers, to South Carolina (COSEWIC 2007). Although previously unknown from Rhode Island, surveys conducted between 1980 and 2005 found it in several locations in the Pawactuck River Basin in southern Rhode Island (Raithel and Hartenstein 2006).

Countries occurrence:

Native:

Canada (Ontario); United States (Connecticut, Delaware, District of Columbia, Maryland, Massachusetts, Michigan, New Hampshire, New Jersey, New York, North Carolina, Ohio, Pennsylvania, South Carolina, Virginia)

A general trend of decline in population abundance has been observed since the first Red List assessment in 1996. In 1988 the Zebra Mussel (Dreissena polymorpha) was only found in a small part of its range and by 1996 it had expanded rapidly to many catchments. At this stage it was suspected that the decline in abundance and loss of range nearly equated to 20%, the threshold for Vulnerable under criteria 2.3.

Since 1996, there has been detailed survey work for most mussel species throughout the north American range as the Zebra Mussel has spread through the region. It is known that this species has been confirmed to be likely extirpated from the main channel of the Detroit River between Lake St. Clair and Lake Erie due to Zebra Mussel invasion (Schloesser et al. 2006). By 2011 the Zebra Mussel was present in most waterways within the range of this species.

In Canada there has been a dramatic reduction in its range since 1996, where over 90% of historical records for the species are in waters that are now infested with Zebra Mussels and therefore uninhabitable. There are two remaining small, widely separated populations, one in the delta area of Lake St. Clair and one in a tributary of the upper St. Lawrence River. Eight sites in the Welland River watershed in Ontario (part of the Niagara Region) were surveyed in 2008; this species was not found, although it had shown up in surveys between 1926 and 1988 (Morris et al. 2012).

A survey of 27 sites in Rondeau Bay (north shore of Lake Erie) in 2014 and 2015 found live individuals, but it is not considered a viable population (Reid et al. 2016). Surveys of Lake St. Clair and Lake Erie in 2011 and 2012 found that the composition of Lake St. Clair has changed little since 1986. However, although whilst L. nasuta is one of the most abundant species in the habitats, there was little sign of recent reproduction, suggesting that the Zebra Mussel is impacting this sub-population (Zanatta et al. 2015).

In USA, there have also been population declines recorded in many states and it is estimated between 25 and 40% of the remaining populations are being impacted based on presence of Zebra Mussels compared with the original range. This species may be gone from the Hudson River Estuary where it was once considered common with average densities in the tidal Hudson River prior to the zebra mussel invasion were 0.08/sq. meter (Strayer et al. 1994) whereas a single individual was found in 2011 (Strayer and Malcolm 2013). In Ohio, "We conclude that the Eastern Pond mussel may be all but gone from the East Branch and West Branch Cuyahoga Rivers, but may persist in better numbers in Portage County" (Andrikanich, Sredniawa and Krebs 2012) and surveys of the Upper Cuyahoga River show declines between 1985 (24 live, 18 dead), 1990 (24 live, 21 dead) and 2012 (18 live, 34 dead) (Krebs et al. 2012). In New York State between 1930 to 1982 this species was the second most common Unionid in western Lake Erie (Nalepa et al. 1991), but by 1991 it had disappeared (Schloesser and Nalepa 1994). In Connecticut and Massachusetts it is listed as special concern as many of its historic populations are thought to be extirpated or in decline, and there are few remaining populations that are considered healthy and stable. Environmental pollution and habitat degradation are considered the primary reasons for the initial decline and Zebra Mussels post 1990. Hence it is considered to be in Delaware: Ligumia nasuta is a Tier 1 Species of Greatest Conservation need for the state. Molecular analyses (mtDNA) by Scott et al. (2014) found that the Great Lakes populations have a lower genetic diversity than the Atlantic coast populations, suggesting that all the Great Lakes populations derived from a single, small founder group. This analysis was based on 64 individuals collected from 17 sites across the five major watersheds (Lake Michigan and Huron drainages, Lake St. Clair, Lake Erie, Lake Ontario (including the St. Lawrence River system), and Atlantic Coast rivers. The Atlantic Coast populations have more diversity and the Zebra Mussels took longer to get into these systems.

The Eastern Pondmussel occurs in sheltered areas of lakes, in slack-water areas of rivers and in canals, where it prefers substrates of fine sand and mud at depths ranging from 0.3 to 4.5 m. In Lake St. Clair, it is currently found on substrates composed of over 95% sand at the transition zone between the emergent wetlands and the open waters of the lake (COSEWIC 2007).

Ligumia nasuta, like those of most other freshwater mussels, are obligate parasites of fishes (COSEWIC 2007).

Ligumia nasuta is a long-term brooder that spawns in late summer, broods its glochidia over the winter and releases them in the spring. The possible host fish for the Eastern Pondmussel are unknown, but the occurrence of this mussel in coastal rivers along the Atlantic seaboard suggests that at least one of the hosts is tolerant of brackish water.

Adult L. nasuta feed on bacteria, algae and other organic particles that are filtered from the water column. Juveniles live completely buried in the substrate and feed on similar food items obtained directly from the substrate or interstitial water (COSEWIC 2007).

Historically, unionids were used for pearl and button industry. Most of this industry occurred from rivers and not prevalent in the historic range of Ligumia nasuta. However, the shells are thick enough that they could have been used at local scales.

Zebra Mussels (aquatic invasive species) have destroyed most of the available habitat for L. nasuta throughout its range and continue to threaten the remnant population in the delta area of Lake St. Clair. Ligumia nasuta in the St. Clair delta occur at depths of 1.5 m or less. Any changes in water levels in lakes, for example, to climate change could also negatively impact the species (COSEWIC 2007). Surveys of Lake St. Clair and Lake Erie found several locations where native mussels continue to thrive in unionid refuges and the composition in Lake St. Clair has changed little since 1986, but there is also little sign of reproduction (Zanatta et al. 2015).

In other parts of the range localised pollution has impacted populations as well as river channel management for dams and dredging shipping channels.

This species is listed as Endangered under SARA (Canada- Federal) although is not currently listed or proposed for listing under the U.S. Endangered Species Act (U.S. Fish and Wildlife Service 2005). It has been designated as Endangered in Ohio and Delaware, Threatened in New Jersey and North Carolina and Special Concern in Massachusetts and Connecticut (COSEWIC 2007).